(a) Field of the Invention
The present invention relates to an image taking system to be used for photographing images, such as TV images, which are formed on screens having curvature.
(b) Description of the Prior Art
As one of the conventional image taking systems of this type, there is known a system so adapted as to photograph TV images with a plural number of cameras having different photographing image sizes which are to be arranged selectively in front of a TV screen displaying images on the basis of video signals.
This system consists, for example as illustrated in FIG. 1, of a TV monitor 1 for displaying images formed on the basis of video signals, a main body 4 comprising an electrical circuit 2 connected to the TV monitor 1 and having mount 3 located in front of the screen of the TV monitor 1, and a plural number of cameras 8, 8' which comprises color correcting filters 5, 5' for matching colors of images formed on the TV monitor 1 with spectral sensitivities of films, imaging lens systems 6, 6' and films 7, 7', are different from one another in size and focal length thereof, and can be attached selectively to the mount of the main body 4. This system is so adapted as to photograph images displayed on the TV monitor 1 by focusing the images onto the films 7, 7' with the imaging lens systems 6, 6' through the color correcting filters 5, 5' as illustrated in FIG. 2A and FIG. 2B.
In the system described above, however, the optical systems of the cameras 8, 8' are not composed systematically and the distances as measured from the imaging lens systems 6, 6' to the TV monitor 1 are different for the individual cameras. Accordingly, the mount for attaching the imaging lens systems to the camera bodies has complicated mechanical structure and diameter of the light bundle passing through the mount is different for the individual cameras. In order to allow all the cameras to be attached commonly to the mount, it is necessary to uniformalize diameters of the mounts and the filters 5, 5' with that of the largest mount, thereby increasing diameters of the mount 3 and the filters 5, 5', and posing a problem to enlarge the image taking system as a whole. In order to solve this problem, there has been proposed an image taking system which comprises, as disclosed by Japanese Preliminary Patent Publication No. 66242/60, a pair of imaging lens systems which can be inserted alternately into the photographing optical path and have focal lengths different from each other, and filters or the similar members arranged at intermediate locations in the individual imaging lens systems. In the composition of this image taking system where the imaging lens systems are exchanged both inside and outside the mounts, however, mechanical structure of the mounts is more complicated and the imaging lens systems cannot be exchanged easily, thereby posing a problem that location of the filters or the similar members is restricted.
Further, the screen of the TV monitor 1 generally has curvature which is classified into the two types shown in FIG. 3A and FIG. 3B respectively. The screen illustrated in FIG. 3A is referred to as the cylindrical type which has the shape of a portion of a cylindrical surface having a radius of R. The screen shown in FIG. 3B is referred to as the spherical type which has the shape of a portion of a spherical surface having a radius of R. Now let us consider a case where an image formed on the TV monitor 1 is to be photographed with a lens system free from aberrations including distortion. FIG. 4A illustrates an image 9' which is obtained by forming a lattice image 9 as shown in FIG. 4C on the cylindrical type screen of the TV monitor 1 having the radius of R as shown in FIG. 4B and photographing the lattice image onto the film 7 with the imaging lens system 6. Let us assume that a reference symbol x represents the distance as measured from the front focal point ff of the imaging lens system 6 to the center of the screen of the TV monitor 1 and that the line 10 of the lattice image 9 is located at the center of the screen. Let us further assume that a reference symbol .DELTA.x represents a difference in the distance measured in the direction along the optical axis from the center of the screen to the outermost circumference, i.e., the line 11. When focal length of the imaging lens system is designated by a reference symbol f, we obtain: EQU Magnification .beta..sub.10 for the line 10=f/x (1) ##EQU1## Due to the difference between the magnification levels expressed by the equations (1) and (2), the lattice image is photographed as the image 9' on the film 7. Speaking more concretely, the image 9 formed on the screen of the TV monitor 1 is affected by the magnification difference produced dependently on image height. When the image 10' of the line 10 is considered as an ideal image, the image 11' of the line 11 is affected by the magnification difference (difference between the actual magnification and ideal magnification) or considered as contracted as compared with the image 10'. This consideration is similar to the concept of distortion in the optical theory which is defined as: ##EQU2## When the equations (1) and (2) are used in the formula (3) above and distortion in this case is represented by DISC (%), we obtain: ##EQU3## In this case, sign of .DELTA.x is taken as positive when it is measured leftward along the optical axis.
Now, let us consider the case of the TV monitor 1 which has the spherical type screen illustrated in FIG. 3B. FIG. 5A shows a front view of the TV monitor 1. The TV monitor 1 generally has a vertical-to-horizontal ratio which is equal to an interlace ratio of 3:4. FIG. 5B shows a sectional view taken along the diagonal direction 12 in FIG. 5A, whereas FIG. 5C shows a sectional view taken along the diagonal direction z in FIG. 5A. When a lattice image 13 is formed on the spherical type screen of the TV monitor 1 by using the imaging lens system 6, magnification for the center line 14 and that for the outermost circumferential line 15 are: ##EQU4## Assuming that .beta..sub.14 is the ideal magnification and 15 is the actual magnification, distortion DISS (%) on the spherical type screen having the radius of R is expressed by the following formula (7): ##EQU5## Wherein the reference symbols .DELTA..gamma. and .DELTA..eta. represent distances measured along the optical axis between the center of the screen and the outermost circumferences, taking the signs thereof as positive when measured leftward along the optical axis. As is understood from the formulae (4) and (7), the spherical type is more advantageous than the cylindrical type when both the types of screen have the same size and the same curvature. In the recent days where color graphics, tables, graphs, etc. displayed on TV monitors are frequently photographed, it is desired to completely eliminate or correct distortion for these purposes. However, it is difficult to correct images so as to be observable in normal shapes thereof by the CRTs proper from the viewpoint of designing of the polarization yokes of CTRs and the images are generally deformed when observed by human eyes.
Moreover, distortion in lens systems allowable or insensible for photographing is generally considered as that within a range from -2% (barrel type) to +1.5% (spindle type). It is therefore desirable to limit distortion within this range also for photographing images formed on the screens of TV monitors.